The present invention relates to glassware formed from spontaneous NaF opal glasses, and is particularly concerned with the treatment of such glassware to improve the surface durability thereof.
In the prior art, a number of different techniques have been developed to improve the surface durability of clear glasses, such as the so-called soda lime glasses, which are the Na.sub.2 O-CaO-SiO.sub.2 glasses widely used for the manufacture of glass containers. One of the oldest methods involves the treatment of the container with sulfur or a sulfur oxide gas, as shown, for example, by U.S. Pat. No. 1,933,529 to Wardley et al. A survey of such methods is provided by Coward and Turner, J. Soc. Glass Tech., 22, 309-319 (1938).
Other processes which have been proposed for improving the durability of glass containers by the treatment of hot glassware include treatment with halogen-substituted hydrocarbons (U.S. Pat. No. 2,261,689 to Lyle et al.), treatment with hot, dry air (U.S. Pat. No. 3,215,515 to Bacon), treatment with fluorine-containing substances (U.S. Pat. No. 3,314,772 to Poole et al.) and treatment by ion-exchange with kaolinitic clays (Surface Dealkalization of Finished Glassware, H. S. Williams and W. A. Weyl, The Glass Industry, 26, 6, 275-301 (1945). A method for treating alkali silicate glasses by sequential exposure to concentrated sulfuric acid and heat is described by le Clerc et al. in U.S. Pat. No. 3,116,991.
A spontaneous sodium fluoride opal glass is a glass which spontaneously transforms from a clear state to an opalized state upon cooling from a melt, wherein sodium fluoride crystals are deemed to constitute the principal opacifying phase. Glasses of this type have recently been considered for tableware and related applications because of the dense, white opacity exhibited thereby. U.S. Pat. Nos. 4,038,448 and 4,080,215 to Boyd et al. disclose examples of spontaneous sodium fluoride opal glasses of sodium aluminosilicate composition intended for such applications, wherein NaF crystals are deemed to constitute the principal opacifying phase. In the glasses of the latter patent, SrF.sub.2 crystals may also be present.
Spontaneous sodium aluminosilicate opal glasses such as above described, comprising NaF crystals as the principal opacifying phase, exhibit reasonably good chemical durability in bulk; however, the surface layer which is formed on glass articles molded from such glass compositions exhibits relatively poor durability. This behavior is presently attributed in part to the presence in the surface layer of excess Na.sub.2 O, termed extractable Na.sub.2 O because it is subject to removal from the surface layer by water or aqueous acidic solutions in much the same way that sodium can be removed from soda lime glasses by appropriate treatment.
The relatively poor durability exhibited by spontaneous NaF opal glasses gives rise to several problems in use, among which are weathering and surface chalking or crazing. Weathering is a reaction between sodium in the glass surface and moisture and CO.sub.2 in the atmosphere. Surface chalking and crazing are due to cracking or spalling of a hydrated surface layer of glass from the article, usually on a microscopic scale, and can be induced in these glasses by the sequential washing and heating of the glassware in use, for example, by ordinary dishwashing and baking procedures.
The susceptibility of opal glassware to this type of behavior is dependent upon both the composition of the glass and the thermal history of the glass article. However, it is very difficult to control both composition and thermal history for the purpose of improving chemical durability, while at the same time retaining the physical properties necessary for the production of dense, white opal tableware.
It is a principal object of the present invention to provide spontaneous opal glassware comprising NaF as the principal opacifying phase which exhibits substantially improved resistance to surface attack by aqueous media while retaining excellent surface quality and the dense white opacity characteristic of such ware.
It is a further object of the invention to provide a method for surface-treating spontaneous sodium fluoride opal glassware to impart substantially improved resistance to surface attack by aqueous media thereto.
Other objects and advantages of the invention will become apparent from the following description thereof.